Abstract: During the 168-hour trial operation of a power plant, a bursting and leakage accident occurred in the secondary superheater. The reasons for the tube explosion were analyzed using methods such as macroscopic observation, geometric dimension measurement, chemical composition analysis, mechanical property testing, metallographic examination, scanning electron microscopy and energy spectrum analysis. The results show that there were two original crack defects on the T91 steel pipe, one of which was a longitudinal crack defect with a length of about 200 mm near the explosion and a depth of about 5.6 mm. Another crack surface was located at a depth of approximately 5.6 mm at the explosion site. During the 168-hour trial operation of the unit, the boiler experienced multiple start-stop and load changes, causing drastic changes in the temperature and pressure of the heating surface, leading to continuous crack propagation and ultimately resulted in a pipe-burst accident.